posix: Ignore non opened files on tst-spawn5

The make program  might open a pipe for its job server, which triggers
an invalid check on the spawned process.  This patch now passes the
lowest file descriptor as ithe first argument, so only the range
that was actually opened is checked.

Checked on x86_64-linux-gnu and i686-linux-gnu and centos7 (which
triggers the issue).